1. Technical Field
This technology relates to ballistic resistant composite articles having improved resistance to backface deformation.
2. Description of the Related Art
The two primary measures of anti-ballistic armor performance are projectile penetration resistance and blunt trauma (“trauma”) resistance. A common characterization of projectile penetration resistance is the V50 velocity, which is the experimentally derived, statistically calculated impact velocity at which a projectile is expected to completely penetrate armor 50% of the time and be completely stopped by the armor 50% of the time. For composites of equal areal density (i.e. the weight of the composite panel divided by the surface area) the higher the V50 the better the penetration resistance of the composite. Whether or not a high speed projectile penetrates armor, when the projectile engages the armor, the impact also deflects the body armor at the area of impact, potentially causing significant non-penetrating, blunt trauma injuries. The measure of the depth of deflection of body armor due to a bullet impact is known as backface signature (“BFS”), also known in the art as backface deformation or trauma signature. Potentially resulting blunt trauma injuries may be as deadly to an individual as if the bullet had fully penetrated the armor and entered the body. This is especially consequential in the context of helmet armor, where the transient protrusion caused by a stopped bullet can still cross the plane of the skull underneath the helmet and cause debilitating or fatal brain damage. Accordingly, there is a need in the art for a method to produce ballistic resistant composites having both superior V50 ballistic performance as well as low backface signature.
It is known that the impact of a high speed projectile with ballistic-resistant armor generates and propagates a compression wave. This compression wave, i.e. a shock wave, propagates outward from the point of impact, causing a transient compression behind the armor. This transient compression often extends beyond the deformation of the armor itself and may be a significant contributor to the resulting depth of backface deformation, causing great blunt trauma. Limiting or mitigating the shock wave energy, or even preventing formation of the shock wave entirely, would effectively reduce the extent of backface deformation.
One method for limiting the effect of a shock wave is by absorbing it. For example, U.S. patent application publication 2012/0234164 teaches a system including a fracture layer comprising an outer ceramic layer, a fracture material that disintegrates into fine particles when it absorbs a shock wave, and a plurality of resonators embedded within the fracture material. The ceramic layer accelerates and spreads out a shock wave generated by a projectile impact, the fracture material absorbs the shock wave which causes it to pump high energy acoustic wave energy, and the resonators reflect this wave energy generated in the fracture layer. This system employs an approach that is counterintuitive to the approach described herein, amplifying the shock wave rather than mitigating it so that the wave has sufficient energy to activate vibrations at particular acoustic spectral line wavelengths.
U.S. patent application publication 2009/0136702 teaches a transparent armor system for modifying the shock wave propagation pattern and subsequent damage pattern of transparent armor such as bullet-resistant glass. They describe the incorporation of a non-planar interior layer positioned between two armor layers. The non-planar interface design of the interior layer modifies the shock wave pattern through geometric scattering and material sound impedance mismatch induced scattering. This type of structure is designed to allow distribution of the impact energy into preferred areas of the armor without causing significant glass shattering and spalling. This system is not directed to body armor.
Other systems are known that employ blast mitigating materials such as aerospace-grade honeycomb materials or blast mitigating foams to suppress shock waves and reduce the impact of high pressure blast energy. Aerospace-grade honeycomb materials are generally characterized as a panel of closely packed geometric cells. It is a structural material that is commonly employed in composites forming structural members in aircraft and vehicles because of their high strength, superior structural properties and versatility, but they are also known for use in ballistic resistant composites. See, for example, U.S. Pat. No. 7,601,654 which teaches rigid ballistic resistant structures comprising a central honeycomb panel positioned between two rigid, ballistic resistant fibrous panels. Blast mitigating foams are useful because they can absorb heat energy from a blast and can collapse and absorb energy by virtue of their viscoelastic properties. Condensable gases in foams may condense under elevated pressure, thereby liberating heat of condensation to the aqueous phase and causing a decrease in shock wave velocity. See, for example, U.S. Pat. No. 6,341,708 which teaches blast resistant and blast directing container assemblies for receiving explosive articles and preventing or minimizing damage in the event of an explosion. The container assemblies are fabricated from one or more bands of a blast resistant material, and are optionally filled with a blast mitigating foam.
These articles of the related art are all limited in their usefulness. They are not optimized for limiting or eliminating shock wave energy while maintaining superior ballistic penetration resistance to high speed projectiles and while also maintaining a low weight that is sufficient for body armor applications. The articles described in both U.S. 2009/0136702 and U.S. 2012/0234164 are heavy, non-fibrous composites that are predominantly used for bullet resistant glass applications. Articles incorporating honeycomb structures are bulky, heavy and not optimized for use in body armor. Articles incorporating blast mitigating foams also have limited effectiveness in body armor applications.
In view of these drawbacks, there is an ongoing need in the art for improved armor solutions that are useful in a wide range of applications, including but not limited to body armor applications. The present system provides a solution to this need in the art.